1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, though not exclusively, to the configuration of a scanning optical device in an image forming apparatus.
2. Description of the Related Art
Known image forming apparatuses will be described with reference to FIGS. 5 to 9.
FIG. 5 shows an image forming apparatus that prints a color image. The image forming apparatus includes independent image bearing members (hereinafter referred to as photoconductive drums) corresponding to yellow, magenta, cyan, and black colors. Each photoconductive drum is composed of a conductor coated with a photosensitive layer, and is included in a corresponding image forming unit. An electrostatic latent image is formed on the photoconductive drum by using laser light emitted from a scanning optical device 21. The scanning optical device 21 emits laser light according to image information sent, for example, from an image reader or a personal computer. Each image forming unit also includes a developing device 22 that forms a toner image on the photoconductive drum with frictionally charged toner. Toner images formed on the photoconductive drums are superimposed on an intermediate transfer belt 23. A sheet cassette 24 is provided at the bottom of the apparatus, and stores recording materials onto which toner images are transferred. A fixing device 25 adsorbs transferred toner images on the recording materials by heat. After fixing, the recording materials are stacked in an output tray 26. Each image forming unit also includes a cleaning device 27 that cleans the corresponding photoconductive drum of residual toner.
An electrostatic latent image is formed on each photoconductive drum charged by a charging device by applying laser light from the scanning optical device 21 onto the photoconductive drum according to image information. Subsequently, toner frictionally charged in the developing device 22 adheres to the electrostatic latent image so as to form a toner image on the photoconductive drum. The toner image is then transferred from the photoconductive drum onto the intermediate transfer belt 23, and is transferred again onto a recording material supplied from the sheet cassette 24. The toner image transferred on the recording material is fixed by the fixing device 25, and the recording material is ejected in the output tray 26.
FIG. 6 shows the scanning optical device 21 shown in FIG. 5. Since the scanning optical device 21 has a bilaterally symmetrical structure, components only in one side of the device are denoted by reference numerals in the figure. In the scanning optical device 21, two laser beams enter each side of a single polygon mirror 28 so that the photoconductive drums are exposed to irradiation light beams E1 to E4. The scanning optical device 21 adopts an oblique incident optical system, and second focusing lenses are arranged so that laser light beams pass therethrough after being separated. Herein, an oblique incident optical system refers to an optical system in which laser light is obliquely incident on the polygon mirror and upper and lower optical paths are separated after the laser light exits from the polygon mirror. When it is assumed that a plane (X-Y plane) defined by the normal to the surface of the polygon mirror 28 and the rotating direction of the polygon mirror 28 is designated as a base plane, as shown in FIG. 7, laser light beams enter the base plane at different angles (these angles will be referred to as oblique incident angles).
Optical components of the scanning optical device 21 are mounted in an optical box 33, and are shielded from the outside by an upper cover 34 that closes the optical box 33. Dustproof glasses 32 are provided in the upper cover 34 so as to protect the scanning optical device 21 from dust.
Two laser beams exiting from the polygon mirror 28 pass through a first focusing lens 29. One of the laser beams traveling on the photoconductive-drum side is reflected downward by a separation folding mirror 31c. The first focusing lens 29 is formed of a cylindrical lens because the laser beams are incident thereon at different angles. The laser beams are focused in the sub-scanning direction by second focusing lenses 30 placed in the optical paths thereof. The laser light beam E2 reflected by the separation folding mirror 31c, crosses the other laser beam E1, and travels away from the photoconductive drum. The laser light beam E2 then passes through one of the second focusing lens 30, and is reflected again by a folding mirror 31b placed on the lower surface of the optical box 33, and then passes beside the first focusing lens 29, and is applied onto the photoconductive drum.
Each of the laser beams E1 and E4 to be applied onto the photoconductive drums, provided at both ends, passes between the separation folding mirror 31c and the folding mirror 31b, passes through one of the second focusing lens 30, is reflected by a folding mirror 31a, and is applied onto the corresponding photoconductive drum. The separation folding mirror 31c is positioned so that two laser beams are not blocked, for example, because of parts tolerances or tilting of a polygon motor.
A description will now be given of the characteristics of the oblique incident optical system. By adopting the oblique incident optical system in the scanning optical device, a plurality of beams can be simultaneously subjected to deflection scanning without increasing the size of the device.
In contrast, in the oblique incident optical system, pitch nonuniformity (hereinafter referred to as tilting) due to decentering of the polygon mirror increases in principle, compared with an optical system in which the oblique incident angle is zero, that is, light beams perpendicularly enter the reflecting surface of the polygon mirror. This results from decentering of the reflecting surface of the polygon mirror from the rotating shaft of the polygon motor.
FIG. 8 shows the path of a light beam near the polygon mirror in the oblique incident optical system. In FIG. 8, a light beam is incident on the polygon mirror 28, which is decentered from the rotating shaft by a distance d, at an oblique incident angle α. Normally, decentering is caused by two factors, namely, variation among polygon mirrors and rattling between the rotating shaft of the motor and the polygon mirror.
When the polygon mirror is decentered from the rotating shaft by d, as shown in FIG. 8, the reflecting surface of the polygon mirror is displaced by d in one rotation of the polygon mirror. In the oblique incident optical system, the reflecting position on the polygon mirror is changed by this decentering, and the light beam deviates in the sub-scanning direction, as shown by a broken line 800 in FIG. 8. As a result, sub-scanning deviation having the same frequency as tilting (rotational frequency of the polygon mirror) occurs. Since tilting due to decentering increases as the oblique incident angle increases, it should be minimized. For this reason, when laser light is scanned by one rotatable polygon mirror in an image forming apparatus including more than four photoconductive drums, an optical path which is not optically symmetrical in the right-left and up-down directions of the rotatable polygon mirror is produced, and the performance of this optical path becomes lower than that of other optical paths.
On the other hand, there is an increasing demand to improve the quality of images formed by image forming apparatuses. For that purpose, image formation has been performed with popular yellow, magenta, cyan, and black toners and other color toners. In order to achieve this type of image formation, an image forming apparatus can include five or more photoconductive drums. Japanese Patent Laid-Open No. 09-146333 discusses an image forming apparatus including five image forming units corresponding to basic colors of yellow, magenta, cyan, and black and to a particular color of matte black, as shown in FIG. 9. In each image forming unit, exposure is performed by a specific scanning optical device.
However, when the image forming unit for forming an image of a particular color is added, the number of photoconductive drums increases, and this tends to increase the size of the image forming apparatus. In contrast, image exposure for a plurality of photoconductive drums can be performed by using one rotatable polygon mirror without preparing an exposure unit corresponding to each of the photoconductive drums. That is, photoconductive drums are provided to form images with yellow, magenta, cyan, and black toners, and electrostatic latent images are formed thereon by image exposure with one rotatable polygon mirror. In this case, in order to apply laser light for image exposure onto a photoconductive drum on which a particular color toner image is formed, it is conceivable that the following configuration can be adopted. That is, laser light is deflected and scanned on photoconductive drums for basic colors of yellow, magenta, cyan, and black and a particular color by one rotatable polygon mirror. This configuration not only can reduce the number of rotatable polygon mirrors, but also can reduce the distance between the photoconductive drums.
In the above-described configuration, however, the oblique incident angles need to be larger than in the case when light is deflected and scanned on four photoconductive drums. Tilting is worsened by the increase in the oblique incident angles, and this can be a serious problem in maintaining the high image quality of the image forming apparatus. While it can be necessary to increase the precision of the rotatable polygon mirror and the precision of the rotating shaft in order to overcome this problem, it is difficult to completely avoid tilting.
In a method shown in FIG. 10, the above-described oblique incident optical system is not used, and the oblique incident angle is zero, that is, laser light beams perpendicularly enter a reflecting surface of a polygon mirror. In this optical system, the laser light beams also need to be separated while traveling toward photoconductive drums, and the vertical distance between the separated laser beams needs to be large. For this reason, when exposure is performed on more than four photoconductive drums by the optical device using one polygon mirror, the thickness of the polygon mirror increases, and this causes problems of, for example, driving load, vibration, noise, and heat generation.